Inherently, the submarine is also subjected to external disturbance forces, affecting the dynamic behavior of the submarine, and represented by a disturbance vector and such disturbance forces are caused to act when the submarine is out-of-trim, that is, heavy or light with respect to the fluid, giving heave force, or unbalanced fore-aft, giving a pitching moment, both of which constitute motion-disturbing out-of-trim forces.
It is known for a submarine to include an autopilot, the autopilot to include a controller arranged to receive, possibly inter alia, input signals or numerical values each representative of any error between the depth and an ordered depth. In response, the controller is arranged to produce a corresponding control signal, this signal determining the desired operating values or positions of controls, the controls including, for example, hydroplane deflections, or thrusters. Thus, there are caused required changes to the motion of the submarine manifested by corresponding changes of the state variables representing the dynamic behavior of the submarine, so that the submarine moves towards an ordered depth and/or maintains a depth obtained. Each control signal represents a so-called control input vector.
Hence, an autopilot can be considered to be a part of a system arranged to operate automatically upon ordered values of depth, and, in response, the autopilot is to provide desired control signals, whereby there tends to be obtained, and/or there tends to be maintained, the ordered depth of the submarine.
For convenience the inputs to and outputs from parts of the autopilot are referred to generically as `values`, irrespective of whether they are numerical values or signal levels.
It is known for the autopilot to include a controller having a linear, or proportional, transfer function and arranged to receive input values each representative of any instantaneous error between the submarine's depth, and an ordered depth; and possibly also input values each representative of the derivative of the depth error, the change of pitch angle, and the derivative of the change of pitch angle. The controller is referred to as a Proportional Derivative Controller. Such a controller operates satisfactorily only when the disturbance to which the submarine is subjected has a zero mean with respect to time. Such a controller does not cause the submarine to keep depth accurately when the submarine is out-of-trim, there then being a constant, or diverging, offset from the ordered depth.
If the controller is caused also to operate upon a term in the integral of depth error to become a Proportional Integral Derivative (PID) controller, the problem, in relation to depth keeping when the submarine is out-of-trim, is overcome, the offset being removed. With such a controller, however, there are difficulties in relation to depth keeping, and pitch keeping, when the submarine is maneuvering, for example, when changing depth.
If the motion of the submarine is expressed in the form of state-space equations, the input to the controller referred to in the preceding paragraphs can be considered to be representative of the instantaneous values of variables comprising the out-of-trim heave velocity, the out-of-trim pitch velocity, the change of pitch, and the depth error, together considered to be represented by a set, or so-called vector, of state error variables. However, the change of heave velocity cannot be measured readily; and the change of pitch velocity requires a rate gyroscope, or other instrument, rendering the system complex and expensive.
Thus, it has been proposed that, advantageously, the autopilot includes a simulator, or state estimator, to calculate, in real time, estimated values of appropriate variables, together considered to be represented by a set of values comprising a so-called vector of estimated state variables. Each value representative of this vector is operated upon, within the autopilot, to provide a corresponding value representative of a so-called vector of estimated state error variables, and these values are supplied to the controller, instead of the values representative of the vector of state error variables, referred to in the preceding paragraph.
In order to overcome the problem, in relation to depth keeping when the submarine is out-of-trim, as referred to above, integration of the depth error, as with the Proportional Integral Derivative Controller, has been proposed for an autopilot including a simulator, or state estimator. However, because with a system designed to operate upon the state-space form of equations of submarine motion, that is, all of the variables of the vector, the controller output is required to be a linear combination of its inputs, the desired integration cannot be performed by the controller. Instead and to achieve the same result, it has been proposed that the autopilot is arranged to operate upon an ancillary state error variable, comprising the integral of depth error, the required integration being performed in a part of the autopilot outside the controller. Such an arrangement is equivalent to the integration, performed by the Proportional Integral Derivative Controller, being introduced into the controller of an autopilot including a state estimator. However, as with the Proportional Integral Derivative Controller, although the problem, in relation to depth keeping when the submarine is out-of-trim, is overcome there remain difficulties in relation to depth keeping, and pitch keeping, when the submarine is maneuvering.
In particular, and in order to permit implementation of the present invention, the arrangement of such a control system with an autopilot having a state estimator includes the output of the state estimator being connected to the controller, via a differencer determining arrangement, hereinafter referred to as differencer, the differencer being arranged, in addition to receiving each output value from the state estimator, also to receive a value representative of at least a function of, that is, related to, an instantaneously ordered depth for the submarine and modifying the appropriate estimated variable value in accordance with the difference between them. In response, the differencer provides to the controller from the modified and unmodified values of estimated state variables corresponding controller input values, representative of the vector of estimated state error variables. This vector is partially representative of any instantaneous error between the estimated, and, in effect, the ordered, values of the submarine's depth, in relation to, at least the function of, the ordered depth. The controller produces, in response, an output in the form of, or translatable to, a control signal to determine the instantaneously desired values of controls of the submarine, that is, the vehicle controls, which include, for example, hydroplanes whose deflections interface with the fluid to define the submarine depth and pitch. This control signal, or rather the component values thereof, is supplied also to the state estimator, to update the values of the vector of estimated state variables computed by the state estimator.
Further, the arrangement conveniently can be considered as including, in the physical part, measurement means connected to observe the dynamic behavior of the vehicle and provide a set of values, signals or numerical data, representative of the instantaneous, observed values of the state variables, together considered as the vector of state observations. Conveniently, the autopilot can be considered also as including the equivalent of such measurement means, connected to the output of the state estimator, in the form of an autopilot observation model which produces a set of values representative of the instantaneously estimated values of the corresponding, observation variables, or the vector of estimated state observations. Each output value from the autopilot observation model is compared with the value of a corresponding variable considered to be from the measurement means of the physical part of the system in a differencer, the output of which differencer is connected to the input of the state estimator so that the estimated state computed by the state estimator is updated in response to each output signal from this differencer. This differencer provides a set of values comprising a vector of observed estimation error variables and represents the instantaneous differences between the observed values, and the estimated observed values, of the observation variables.
In accordance with the generality of the problem to all fluid born vehicles it is desirable, and comprises an object of the present invention, to provide, for a fluid borne vehicle a control system including an autopilot, with a simulator state estimator, which causes the vehicle to assume a steady or state condition in relation to state variables observed at which motion-disturbing out-of-trim forces acting on the vehicle are compensated for, novel means to compute, in real time, any out-of-trim heave force, and any out-of-trim pitching moment, either acting on, or to act on, the vehicle.